Whiterockite, CaMgMn³⁺₃O₂(PO₄)₂CO₃F·5H₂O, a new phosphate–carbonate mineral from the White Rock No.2 quarry, South Australia, Australia

Abstract

Whiterockite, CaMgMn³⁺₃O₂(PO₄)₂CO₃F·5H₂O, is a new phosphate–carbonate mineral from the White Rock No. 2 quarry, Bimbowrie Conservation Park, South Australia, Australia. The mineral is associated with dufrénite/natrodufrénite, ushkovite, bermanite, leucophosphite and sellaite in a matrix comprising fluorapatite and minor quartz. Whiterockite has formed from hydrothermal alteration and weathering in an oxidising, low-temperature and low-pH environment. Whiterockite forms aggregates of thin platy dark-red crystals up to 0.7 mm across with individual crystals up to 0.2 mm in across. Crystals are transparent with a vitreous lustre. The mineral is brittle, has a perfect cleavage on {001} and has an irregular fracture. The measured density is 2.76(2) g/cm^–3. Whiterockite is optically biaxial (–), with α = 1.660(3), β = 1.760(5) and γ = 1.770(5), determined in white light; 2V_meas = 30(1)°; and orientation: X ≈ c*. The mineral is pleochroic: shades of red brown; X < Y < Z. Electron microprobe analyses provided the empirical formula (Ca_0.87Na_0.18)_Σ1.05Mg_1.05(Mn³⁺_2.87Fe³⁺_0.10)_Σ2.97O_1.93(PO₄)_2.01CO₃F_1.04·4.99H₂O. Whiterockite is monoclinic, C2/m, a = 11.112(2), b = 6.4551(13), c = 10.667(2) Å, β = 102.61(3)º, V = 746.7(3)ų and Z = 2. The crystal structure of whiterockite has been refined using single-crystal synchrotron X-ray diffraction data to R1 = 5.10% on the basis of 957 reflections with F₀ > 4σ(F₀). The structure can be described as a layered structure formed by the stacking along [001] of three kinds of layers and is related to the structure of jörgkellerite.

Introduction

More than 70 pegmatite bodies are known to occur in the Olary Province of South Australia, many of which are rare-element pegmatites enriched in lithophile elements and characterised by abundant beryl and apatite and additional minerals such as columbite, samarskite, Nb-rutile, phlogopite and triplite–zwieselite (Lottermoser and Lu, 1997). The White Rock pegmatite and the nearby Wiperaminga Hill pegmatite were minor producers of feldspar, beryl and muscovite from the 1930s to the 1970s (Olliver and Steveson, 1982; 1984). Both are enriched in phosphate and have yielded six new phosphate minerals between them. In the present paper we describe the new mineral whiterockite from the White Rock No. 2 quarry. The new mineral was found initially on a specimen collected from the locality in the 1960s. A visit in 2018 was able to locate several additional specimens. The mineral and name (symbol Wrc) have been approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (IMA2020-044, Elliott and Kampf, 2020). The holotype specimen is housed in the mineralogical collection of the South Australian Museum, Adelaide, South Australia, Australia (registration number G34889).

Occurrence

The White Rock pegmatite is located in the Bimbowrie Conservation Park, 24 km N of Olary, South Australia, Australia (140°19ʹE, 32°4ʹS). It is a poorly outcropping beryl–columbite phosphate rare-element type pegmatite in the classification of Černý (1991). The pegmatite is hosted by rocks of the Willyama Supergroup which comprise upper greenschist to amphibolite grade metamorphosed and strongly deformed sedimentary and minor igneous rocks (Lottermoser and Lu, 1997) that are overlain unconformably by late Proterozoic Adelaidean metasediments. The pegmatite was first mined for feldspar and beryl in 1932 (Olliver and Steveson, 1982) and three quarries operated intermittently up until 1973. Recorded production is 860 tonnes of feldspar and 8.1 tonnes of beryl (Olliver and Steveson, 1984; Crooks and Abbot, 2004).

The petrogenesis of rare-element pegmatites in the Olary Block has been described in some detail by Lottermoser and Lu (1997). The pegmatites are mineralogically zoned and characterised by the occurrence of late-stage phosphate nodules between the quartz core and intermediate feldspar-rich zone. Triplite–zwieselite was formed by metasomatic alteration of magmatic fluorapatite and has been transformed by hydrothermal alteration and weathering, in an oxidising, low-temperature and low-pH environment, to give a complex, microcrystalline intergrowth of secondary phosphate minerals. At White Rock, triplite and associated secondary phosphate minerals have been exposed in only the No.2 quarry. Secondary phosphate minerals include bermanite, bimbowrieite, cyrilovite, jahnsite-(NaFeMg), jahnsite-(NaMnMg), magnesiobermanite, mitridatite, perloffite, phosphosiderite, strunzite and ushkovite/laueite. Whiterockite is found in cavities in a matrix comprising fluorapatite and minor quartz. Associated minerals are dufrénite/natrodufrénite, ushkovite, bermanite, leucophosphite and sellaite.

Appearance, physical and optical properties

Whiterockite occurs as aggregates of crystals up to 0.7 mm across (Fig. 1). Individual crystals are thin six-sided plates up to 0.2 mm in width with a thickness of ∼1–2 μm. Based on the crystal structure, the dominant crystal form is probably {001}. The mineral is dark red with a pink streak. The lustre is vitreous and thin crystals are transparent. The Mohs hardness is ∼3 based on scratch tests. Whiterockite is brittle with an irregular fracture and one perfect cleavage on {001}. The density measured by flotation in a mixture of methylene iodide and toluene is 2.76(2). The calculated density using the empirical formula derived from the analytical data is 2.756 g/cm^–3. Whiterockite is optically biaxial (–) with indices of refraction α = 1.660(3), β = 1.760(5) and γ = 1.770(5) measured in white light. The 2V measured using extinction data analysed with EXCALIBR (Gunter et al., 2004) is 30(1)°; the calculated 2V is 33.5°. Dispersion was not observed; however, crystals provided poor conoscopic figures. The optical orientation, intuited from the structure, is X ≈ c*. Pleochroism is X < Y < Z in shades of reddish brown. The Gladstone–Dale compatibility, 1–(K _P/K _C) (Mandarino, 2007) is –0.052 (good), using the empirical formula.

Figure 1. Crystal aggregate of whiterockite. The scale bar is 20 μm in length. Holotype specimen: South Australian Museum registration number G34889.

Chemical data

A crystal aggregate of whiterockite was analysed using a Cameca SXFive electron microprobe operating in WDS mode with an accelerating voltage of 20 kV, beam current of 20 nA, and a 5 μm beam diameter. Data were reduced using the φ(ρZ) method of Pouchou and Pichoir (1991). Because insufficient material is available for direct determination of H₂O and CO₂, they are calculated based upon the structure determination (1 C and 19 anions pfu). Infrared spectroscopy confirmed the presence of H₂O and CO₃ groups. Analytical data are given in Table 1. The empirical formula is (Ca_0.87Na_0.18)_Σ1.05Mg_1.05(Mn³⁺_2.87Fe³⁺_0.10)_Σ2.97O_1.93(PO₄)_2.01CO₃F_1.04·4.99H₂O. The ideal formula is CaMgMn³⁺₃O₂(PO₄)₂CO₃F·5H₂O, which requires CaO 9.04, MgO 6.50, Mn₂O₃ 38.18, P₂O₅ 22.88, CO₂ 7.10, F 3.06, H₂O 14.53, O=F –1.29, total 100 wt.%.

Table 1. Compositional data for whiterockite

* CO₂ calculated from the refined formula.

** H₂O calculated from the refined formula.

S.D. – standard deviation

Infrared spectroscopy

The infrared spectrum of whiterockite (Fig. 2) was obtained from a powdered sample using a Nicolet 5700 Fourier-transform infrared (FTIR) spectrometer equipped with a Nicolet Continuμm IR microscope and a diamond-anvil cell. The presence of a CO₃ group in whiterockite is confirmed by the presence of a strong band at 1458 cm^–1 due to the ν₃ vibration of CO₃^2– groups, together with the ν₂ vibration of CO₃^2– groups at 820 cm^–1. The spectrum shows a broad band centred at 3520 cm^–1 that is attributed to OH stretches and a band at 1620 cm^–1 due to H–O–H bending of H₂O groups. The PO₄ band occurring at 1107 cm^–1 can be assigned to the ν₃ antisymmetric stretching mode and the bands at and 974 and 746 cm^–1 to the ν₁ symmetric stretching mode.

Figure 2. The FTIR spectrum of powdered whiterockite.

X-ray crystallography and structure determination

Powder X-ray diffraction (XRD) data for whiterockite were obtained using a Rigaku R-AXIS Rapid II curved-imaging-plate microdiffractometer, with monochromatised MoKα radiation (50 kV and 40 mA). A Gandolfi-like motion on the φ and ω axes was used to randomise the sample. Observed d values and intensities were derived by profile fitting using JADE Pro software (Materials Data, Inc.). Data (in Å for MoKα) are given in Table 2. Unit-cell parameters refined from the powder data using JADE Pro with whole-pattern fitting are: a = 11.121(10), b = 6.454(11), c = 10.601(10) Å, β = 101.33(3)° and V = 746.1(16) Å³.

Table 2. Powder X-ray diffraction data for whiterockite

A crystal was attached to a MiTeGen polymer loop and XRD data was collected at the micro-focus macromolecular MX2 beamline at the Australian Synchrotron, part of ANSTO (Aragao et al., 2018). Intensity data were collected using a Dectris EigerX 16M detector and monochromatic radiation with a wavelength of 0.710756 Å. The data was integrated in P1 using XDS (Kabsch, 2010) and absorption correction was carried out with SADABS (Bruker, 2001). The structure was solved in space group C2/m using SHELXT (Sheldrick, 2015a) within the WinGX program suite (Farrugia, 2012) and refined using SHELXL-2018 (Sheldrick, 2015b). The Ca and Mg sites refined to approximately half occupancy with refined occupancies of 0.476(6) and 0.641(11), respectively. The refined site scattering at the Mg (15.3 electrons) is greater than the value from the chemical analysis (12.6 electrons) which suggests that the crystal used for single-crystal XRD may contain more Fe or Mn than that used for the chemical analyses. Two of the seven O sites, O4 and O5, associated with the CO₃ group, also refined to approximately half occupancy, with refined occupancies of 0.520(13) and 0.51(2), respectively. The bond-valence sum for the OW8 anion (0.80 valence units, calculated using the parameters for O) lies between the values expected for donor atoms of F (∼1.0 vu) and H₂O molecules (∼0.5 vu) indicating a mixed occupancy of the site. The chemical analysis indicates ∼50% occupancy of the site by F, so the site was refined with joint occupancy by O and F. The H atom locations could not be found in the difference-Fourier maps. Refinement employing anisotropic displacement parameters converged at R ₁ = 5.10% for 957 observed reflections with F _o > 4σF _o. Details of the data collection and structure refinement are provided in Table 3. Fractional coordinates and anisotropic atom displacement parameters are given in Table 4, selected bond distances are reported in Table 5 and bond-valence values are given in Table 6. The crystallographic information files have been deposited with the Principal Editor of Mineralogical Magazine and are available as Supplementary material (see below).

Table 3. Crystal data, data collection and refinement details

^† wR2 = Σw(|F _o|²–|F _c|²) ²/Σw|F _o|²)^1/2; w = 1/[σ²(F _o²) + (0.0787 P)² + 7.16 P];

P = ([max of (0 or F ₀²)] + 2F _c²)/ 3

Table 4. Fractional coordinates and displacement parameters (Ų) for atoms for whiterockite

^a Refined occupancy 0.476(6); ^bRefined occupancy 0.641(11); ^cRefined occupancy 0.520(13); ^dRefined occupancy 0.51(2); ^eRefined occupancy O_0.5F_0.5

Table 5. Selected interatomic distances (Å) for whiterockite

Table 6. Bond valence analysis for whiterockite

M–O bond-valence parameters are taken from Gagné and Hawthorne (2015), M–F bond-valence parameters are taken from Brown and Altermatt (1985).

Bond-valence sum for the O7 site is based on the occupancy [(H₂O)_0.5,F_0.5].

¹ O2 bonded to Ca, Mn1 and P; ² O2 bonded to Mn1 and P; ³ O3 bonded to Ca, Mn2 and P; ⁴ O3 bonded to Mn2 and P; ⁵ O6 bonded to Ca, 2 x Mn1 and Mn2; ⁶ O6 bonded to 2 x Mn1 and Mn2; ⁷ OW7 bonded to Ca and Mg; ⁸ OW7 bonded to Ca; ⁹ OW7 bonded to Mg; ¹⁰ OW7 bonded to no cations; ¹¹ OW8/F bonded to Ca and Mg; ¹² OW8/F bonded to Mg.

Description of the structure

The structure of whiterockite contains six cation sites. The Ca site is surrounded by seven O atoms in an augmented octahedral arrangement with a <Ca–O> distance of 2.397 Å. The Mg, Mn1 and Mn2 sites are each surrounded by six anions in distorted octahedral coordinations.

The Mn1 and Mn2 sites exhibit four short equatorial Mn–O bonds and two long apical Mn–O bonds. Such [4+2]-distortion is typical of the Jahn-Teller distortion shown by trivalent Mn. The structure consists of three distinct layers stacked along [001] in the sequence shown in Fig. 3: (1) a layer composed of Mn octahedra and CO₃ groups in which Mn1 and Mn2 octahedra share edges to form layers in which every fourth Mn site is vacant and replaced by CO₃ groups (Figs 4 and 5) resulting in layers of composition [Mn₃O₈(CO₃)]; (2) a layer formed by half-occupied Ca sites and P tetrahedra, which link by corner-sharing; and (3) a layer containing half-occupied Mg-centred octahedra. The Mn layer is flanked on either side by Ca/P layers with Ca polyhedra sharing two edges with Mn octahedra and P tetrahedra sharing corners with Mn octahedra. Two Ca layers link via corner-sharing Mg octahedra.

Figure 3. The crystal structure of whiterockite viewed along [010]. The unit cell is outlined. All structure drawings were completed using ATOMS (Dowty, 1999).

Figure 4. The [Mn₃O₈(CO₃)] layer in the structure of whiterockite. The unit cell is outlined.

Figure 5. Fragment of the [Mn₃O₈(CO₃)] layer in the crystal structure of whiterockite. The O4 and O5 sites are half occupied.

Whiterockite is structurally related to jörgkellerite, Na₃Mn³⁺₃(PO₄)₂(CO₃)O₂·5H₂O, a layered phosphate–carbonate mineral from the Oldoinyo Lengai volcano, Gregory rift, northern Tanzania (Zaitsev et al., 2017). Whiterockite is monoclinic, whereas jörgkellerite is trigonal, space group P $\bar 3$ with a significantly different powder XRD pattern. Both structures consist of three distinct layers and the layer sequence in both structures is identical. Both structures contain a topologically identical [Mn₃O₈(CO₃)] layer. In jörgkellerite, this layer links to a layer comprising [7]-coordinated Na polyhedra (84% occupied) and PO₄ tetrahedra, which in turn links to a layer comprising two distinct [8]-coordinated Na polyhedra (each half occupied).